usort() function producing bad results on a linux environment, ok on windows

This is really weird, the same code works on windows, but not on linux.

When I run the code below on a windows machine with PHP 5.2.5, it works perfect, I get the output array alphabetically ordered.

When I run it on a linux box running PHP 5.1.6 I get an array that doesn't have the same order as the original array, but it is also not properly sorted.

Do you guys know what part of my code is "environment dependant"? I can hardly believe the same code will produce two different results in different platforms...

// The code here:
 
class TestTwo{
 
                      // This is just the contructor / driver:
	function TestTwo(){
 
                                          // bootstrapping the input array:
		$sort_array = file_get_contents('test_array_sort.txt');
		$attributes_array = unserialize($sort_array);
 
		$this->attributes_array_sorted = $this->createAttributesArraySorted($attributes_array);			
                                          // At this point, I get the right output on Windows, and the badly sorted array in Linux
	}
	
	private function createAttributesArraySorted($attributes_array_){
		
		foreach($attributes_array_ as &$val){
 
			usort($val['children_nodes'], 'TestTwo::cpm_node');
 
		}
		return $attributes_array_;
		
	} /* end private function createAttributesArraySorted($attributes_array_) */
	
    /*
     * Auxiliary function used to sort attributes in alphabetical order 
     */
    static function cpm_node($a,$b){
    	
    	return strcasecmp($a['node_name'], $b['node_name']);
 
    } /* end static function cpm_nodes() */
    
} /* end class TestTwo */

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